Measurements of the rate of ventilation were made on 312 occasions in thirty-one rooms in (a) a block of older flats, (b) a block of modern flats, (c) three villa residences, and (d) a modern university building.

The rate of air change was determined by liberating a quantity of coal gas into the room and measuring the rate of its subsequent disappearance by means of a katharometer.

The effects on natural ventilation of air gratings and flues were studied in unheated rooms in two blocks of flats. The effective orifices of the gratings were 10·5 sq. in. in the modern flats and 6 sq. in. in the older ones. With the air gratings or flues (if any) closed the rates of air change were 0·55 and 0·71 per hour respectively. With the air gratings open the air change was increased to 0·83 per hour in the modern flats and to 1·04 per hour in the older ones.

The flues in the modern flats in connexion with gas fires were 4 in. diameter pipes (cross-sectional area, 12·6 sq. in.); in the older flats the ordinary fire-places were connected to standard flues and the area of the register openings was 53 sq. in. When the flues were sealed, the air changes per hour in the modern and older flats were 0·84 and 0·72 respectively and when the flues were opened these rates of ventilation were increased to 1·17 and 2·06 air changes per hour. In nine rooms in three other houses the average ventilation rates with the flues closed and open were respectively 0·82 and 1·76 air changes per hour.

The ordinary chimney flue, even without a fire at its base, was an efficient ventilator, but the flue of smaller dimensions was far less effective.

With windows open almost any required amount of ventilation could be obtained, and the observations showed that even when windows are opened only slightly the ventilation is distinctly increased. Thus, in flueless rooms of the modern flats, with doors and windows closed, but with the air grating open, the ventilation amounted to 0·83 air changes per hour, but with one casement only open to a width of 3/8 in. it was increased to 1·79 air changes.

The effects of various factors on the rate of air change are discussed and illustrated. Of these, outside wind speed appears to have most influence. The evidence regarding the effect on ventilation of the difference in temperature between the room air and the outside air is very conflicting. In the present observations, when outside wind speed is taken into account, no definite effect of temperature difference on air change is seen. But the experiments were made in unheated rooms and the temperature difference rarely exceeded 10° F., and in the majority of cases was much less.

The various recommended standards of air supply are discussed. Although certain bye-laws require a fresh-air supply of 1000 cu. ft. per person per hour, it is probable that where there is no overcrowding, as in the normally occupied dwelling-room, a ventilation rate of not less than 600 cu. ft. per person per hour should be sufficient to prevent body odours from becoming apparent.

The results of this inquiry show that in closed, flueless rooms, the air supply is likely to be much below 600 cu. ft. per hour per occupant.

On account of the low ventilation rates observed in flueless rooms it is desirable that the question of the use of flueless gas heaters in such rooms be reconsidered.